Liquid medicament is often injected into the body, where it afterwards is absorbed by the circulatory system. In most cases some kind of syringe is used for this purpose. As there is a multitude of different liquid medicaments, with various viscosity, concentrations and greatly varying dose volumes, within a span from about 5 micro liters doses up to about 800 micro liters doses, there is a multitude of specially designed injectors today, manually operated or equipped with drive mechanisms, which all are adapted for different operation modes. The present invention is designed for, with modest or with no modifications, to be applicable for most of occurring operation modes where an injector with an adjustable dose setting arrangement is used.
Most injectors have in common that they are either equipped with an injection needle which is fixed in the medicament container or equipped with a separate injection needle which is described in the international standard ISO 11608-2, where the exchangeable needle can be screw fitted onto the injector or syringe. In the text below “front side” or “front” refer to the end of the injector which contains the injection needle. “rear side” or “back” refer to the opposite end, which in most cases is composed of a dose knob as in the accompanying illustrated embodiment, or by another arrangement for setting the dose volume which the user intend to inject.
The simplest embodiment of a syringe is a completely manually operated syringe, which most important constituent elements are a tubular needle, which is referred to as the injection needle, a transparent medicament container which in most cases is transparent and equipped with a graded scale which purpose is to indicate the amount of medicament which has been injected into the body. At the upper portion of the syringe there is a rod, referred to as plunger rod, which is equipped with a thumb grip at the top, to make it possible for the user to apply a pressure onto the plunger rod. At the front end the plunger rod is attached to a piston, referred to as plunger or stopper which runs inside of the medicament container. By applying a pressure on the plunger rod, the plunger moves forward in the medicament container, whereby the enclosed medicament passes out through the tubular needle.
A regular type of syringes are the ones known as insulin pens (pen-injector). These are, among other patients, also used by diabetics, some times on several occasions per day. In most cases the insulin is kept in a medicament canister, also known as a cartridge. A cartridge is a glass- or plastic canister which in the front end is sealed by a special cap. This cap comprises a hole in it's front end which normally is sealed by a elastomeric cloth septum) mounted on the inside of the cap. Standardized cartridges are very commonly used by the pharmaceutical industry and often apply to the international standard ISO 11608-3 which also facilitates the FDA-approval of the finished product. There are several companies which produces pen-injectors, which results in that the pen-injector could be designed in a multitude of ways. Nevertheless there is an international standard, ISO 11608-1 which describes certain standard demands which a pen-injector must fulfil, and furthermore this standard states guide lines for the design of the pen-injector. This standard also puts minimum requirements on which quality and repeatability regarding the dose setting that the pen-injector has to fulfil. Typically regarding a pen-injector is that the user sets the requested dose volume by operating a dose knob which is placed on the back end of the pen-injector body, whereby the set dose volume is displayed as a numerical value in a window placed somewhere along the pen-injector body. Also typical for many of the manually operated pen injectors is that the dose knob, when it is turned, protrudes backwards out of the pen-injector body with a turning movement. As the proper dose value is set, which is verified by that the correct numerical dose value is displayed in the window, the user penetrates the injection site
on the body, for example the thigh or the stomach, with the injection needle. Thereafter the user pushes his/her thumb against the dose knob whereby the dose knob is pushed back into the pen-injector body again. As the dose knob is directly or indirectly mechanically connected to the plunger rod, thus the preset volume of liquid medicament is injected into the body. A verification of that the injection is completed is that the dose scale in the window again indicates start position, which can be indicated by a “0” or by another symbol.
There is a multitude of liquid medicaments, and insulin is a typical example of a liquid medicament. Other examples of liquid medicaments are growth hormone or FSH, which is used to support human reproduction. Botox® is yet another example of a liquid medicament. All medicaments consist of unique chemical compositions, concentrations and characteristic dose sizes measured in for example μl (micro liters) or some other dose scale, depending on how the dose volume is represented.
In some cases the pharmaceutical manufacturer states a weight unit for the active substance which can be dissolved in a liquid and by a varying concentration. Therefore, in this case it is the amount of active substance which decides how the measurement of the dose will be represented. Furthermore the viscosity fluctuates between different liquid medicaments. As an example Botox® has a high viscosity.
The viscosity is also in many cases temperature dependent, which results in that medicaments stored in a refrigerator are more viscous than the same medicament stored in room temperature.
In many cases a single medicament container (cartridge) is used for repeated treatments. As an example a cartridge with a volume of 3 ml. containing insulin is sufficient for up to one months use, depending on the size of the doses the patient need to inject and how often the doses are administered. Because of the risk of bacterial growth on the needle which can spread into the medicament through the needle duct, a system of exchangeable needles is used for insulin. It is recommended to change the needle after each injection. The same system with exchangeable needles is also used for many other types of medicaments.
The variable dose sizes result in that the rear portion of the dose knob (which will be pushed into the injector body) will move to different positions behind the injector body when the dose is set, which corresponds to the present dose volume, i.e. the pitch fluctuates.
The patient experiences more or less pain by pricking him/herself with an injection needle, which also, if needles are frequently used, can result in tissue damages at the injection site, which also is well known among diabetics, and therefore the needle manufacturers continuously strive to develop thinner needles.
Even if it's stated that new, thinner injection needles hold the same flow resistance as older, thicker injection needles, practical tests show that the flow resistance tend to increase, the thinner the cross section area gets on new and on future injection needles. Practically, this means that a higher force must be applied on the plunger rod when using future thinner injection needles in order to expel medicament from the medicament container. In those cases where the patient has a restricted nimbleness or strength in his/her hand this is regarded as a limitation. It is often strived for that the patients shall administer the injection of the medicament on their own. As an example, this could result in that children with small hands who inject a large dose of growth hormone experience outright ergonomic difficulties, if they use one hand to grasp around the injection site, that is the body tissue, between one hands thumb and pointing finger, and with the other hand push in the dose knob. This is also an argument for that automatic devices designed for distribution of liquid medicament, which use a
mainspring, electric motor or gas container or some other type of accumulated source of power for driving a plunger rod, will increase their market share on the expense of manually driven plunger rods.
A third type of pen-injectors is a pen-injector where the user, by turning a dose knob, which in most cases is placed on the rear end of the injector body, sets the desired dose volume to be distributed, and simultaneously, through the dose knob applies a load on the spring, which in it's turn propels the plunger rod as the medicament is injected into the body. An example of this is the Autopen (trademark).
Usually occurring sources of power for driving a plunger rod are the ordinary helical compression springs made of steel wire with spring characteristics, torsion springs or clock springs, which are either thin- or narrowly tied clock springs (known as motor springs) of different versions. These springs have in common that they work within an interval and that these springs are stored in a preset state. The reason for this is that through the whole injection course, there must be sufficient power in the spring to overcome the internal power losses such mechanical friction, flow resistance and also the maximum allowed time for an injection.